io_uring/register: drop io_register_enable_rings() submitter_task check

io_uring/register: drop io_register_enable_rings() submitter_task check

io_register_enable_rings() checks that the io_ring_ctx is
IORING_SETUP_R_DISABLED, which ensures submitter_task hasn't been
assigned by io_uring_create() or a previous io_register_enable_rings()
call. So drop the redundant check that submitter_task is NULL.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>